1. Field of Invention
The invention relates to a method of fabricating a poly-silicon (poly-Si) thin film and, in particular, to a method of fabricating a poly-Si thin film with super lateral growth crystals with applications in poly-Si thin film transistors (TFT) or devices with poly-Si thin films.
2. Related Art
The poly-Si has superior electrical properties than the amorphous silicon (a-Si) and the advantage of lower cost than the single silicon crystal. Therefore, it has received a lot of attention in the field of TFT productions recently. It is particularly important in the application of TFT liquid crystal displays (LCD).
However, the grain size of the poly-Si has a great influence on the electron mobility and device properties. The grain boundaries existing in the poly-Si, in particular, are the obstacles when carriers in the device pass through the gate channel. Therefore, how to enlarge the grain size of the poly-Si and thus reduce the number of grain boundaries for enhancing the TFT device properties is an important trend in the poly-Si thin film fabrication technology nowadays. Take the display technology as an example. It is imperative to produce highly-efficient TFF's in order to develop better flat-panel displays.
The conventional method of fabricating the poly-Si thin film is called the solid phase crystallization. However, since the highest temperature that a glass substrate can tolerate is only 650° C. This method is not suitable for making flat-panel displays. Another method is to vapor-deposit the poly-Si thin film directly. Nevertheless, the grain size of the poly-Si formed using either method is very small, at about the order of 100 nm. Consequently, the properties of the poly-Si thin films formed in these ways are not perfect.
Currently, the most commonly used method in fabricating the poly-Si thin film is the excimer laser annealing technology. Although a grain size of about 600 nm can be achieved in this case, it is still insufficient for making high-performance flat-panel displays. Therefore, in recent years, the sequential lateral solidification (SLS) technology has been proposed. Photo masks are utilized to define the pattern of the laser beam, so that the laser irradiates specific parts of the silicon layer, thereby inducing super lateral growth crystallization. By moving the substrate, the laser beam can repeatedly fall on the lateral crystallization region of the poly-Si layer for the crystal grains to grow continuously.
According to the SLS technology, the moving distance of the laser beam has to be smaller than the lateral growth distance of the crystals in the silicon layer, so that the laser irradiated positions fall right on the poly-Si grains of the previous lateral growth crystallization for the grains to continuously grow. However, using the normal laser crystallization techniques that are currently available, the precision of the laser irradiated positions has to relatively high, usually at the order of sub-microns.
Take the dot-SLS technology as another example, shown in FIG. 1B and FIGS. 2A to 2H. FIG. 1 shows the photo mask pattern for the laser beam in the conventional dot-SLS technology. FIGS. 2A to 2H are schematic top views of the crystallization steps in the conventional dot-SLS technology.
In FIG. 1, the dot photo mask 111 is used to block the passage of the laser beam. The region 122 that is not blocked by the dot photo mask 111 is where the laser beam can pass through during the laser crystallization process. Please refer to FIGS. 2A to 2H for an explanation of crystallization process using the current dot-SLS technology. In FIG. 2A, the region 220 represents the position where the silicon layer is actually irradiated by the laser beam. The dot region 210 represents the position where the silicon layer is not irradiated by the laser beam due to the dot photo mask. As the initial dot photo mask covers the central position of the region irradiated by the laser beam, the crystallization in the silicon layer starts from the border of the dot region 210 at the center of the laser beam irradiated region blocked initially by the dot photo mask. The crystallization continues in the radial direction, with a lateral growth crystallization length of about 2 μm. The pattern of the grain boundaries 205 shows the crystallization direction and structure of the poly-Si.
Afterwards, the irradiation position of the laser beam is first moved, as shown in FIG. 2B. The dot region 210 is moved to the position 212 (dashed line), so that the dot region 210 is moved within the range of any lateral crystallization for second laser irradiation. Therefore, outward sequential lateral crystallization of the Si crystal layer starts from the outer border of the position 212. The grain grows in such a way to eliminate some of the grain boundaries 205, forming the crystal state illustrated in FIG. 2C.
Afterwards, the irradiation position of the laser beam is moved for the second time, so that dot region 210 is moved from the position 212 to the position 214, falling within the range of another lateral crystallization, as shown in FIG. 2D. A third laser irradiation is performed to further remove some grain boundary 205, forming the crystal state shown in FIG. 2E.
If the irradiation position of the third laser beam is further moved so that the dot region 210 moves from the position 214 to the position 216, there is a very large probability for the dot region 210 to fall right on the position without any grain boundary in addition to any poly-Si lateral crystallization, as shown in FIG. 2F.
Finally, a fourth laser irradiation is performed to further remove the grain boundary 205. Therefore, one obtains a crystal state with no grain boundary as shown in FIG. 2G. Using this dot-SLS technique can possibly reach a crystal grain close to a single crystal structure as shown in FIG. 2H.
However, since the size of the dot region is usually about 1.5 μm in diameter but each moving of the dot region has to fall within the range of poly-Si lateral crystallization, i.e., 2 μm, the precision in moving the laser beam has to achieve the sub-μm level. Only with such a precision can one obtain a poly-Si grain with a size close to 5 μm after four times of laser irradiation (FIG. 2H).
Therefore, using the SLS technique or the dot-SLS technique, the production will be limited by the strict requirement in the moving precision of the substrate or the laser beam. This will greatly increase the machine cost and lower the yield. It is particularly difficult for large-area panel production. Therefore, even though the SLS technique can improve the current laser crystallization technique, it is still not suitable for the mass production of poly-Si devices.